I travel a 3 class plane sold as 2 classes on the ORD-SFO route every week. I purchase my ticket one week in advance and never have any trouble reserving one of the "enhanced" coach seats at the time of purchasing my ticket. It is my understanding that these seats are held for gold/plat/exp passengers only. I can't speak for the EWR-LAX route, but I suggest you just check the seat map on the web 10 days out, 5 days out, and 3 days out. Do this for one or two weeks and you'll get a good feel for the availability at each point in tim.